The implementation of FileDialog will be a platform file dialog if possible. If that isn't possible, then it will try to instantiate a QFileDialog. If that also isn't possible, then it will fall back to a QML implementation, DefaultFileDialog.qml. In that case you can customize the appearance by editing this file. DefaultFileDialog.qml contains a Rectangle to hold the dialog's contents, because certain embedded systems do not support multiple top-level windows. When the dialog becomes visible, it will automatically be wrapped in a Window if possible, or simply reparented on top of the main window if there can only be one window.

class Struct {
    Struct() = default;
};

#include <iostream>

struct Test {
    int a, b, c;
};

int main() {
    Test t {1, 2, 3};
    std::cout 
        << t.a << " " 
        << t.b << " " 
        << t.c << std::endl;
    return 0;
}